The present invention pertains to that type of dental chair known in the art wherein the articulation of the backrest with respect to the seat of the dental chair is accomplished without relative translation of the backrest with respect to the head of the patient. In this respect, it is desirable to have the backrest pivoted with respect to the seat at some location simulating the theoretical hip joint of the patient. Without such an arrangement, the curve as traced by a point on the back of the patient's head during articulation of the backrest does not coincide with a curve traced by a corresponding point on the headrest of the chair. Thus, the headrest appears to translate with respect to the patient's head during inclination and causes discomfort to the patient.
Several arrangements are known for pivoting the backrest with respect to the seat which eliminates this relative movement. For example, the backrest may be actually pivoted to a bracket upstanding from the seat so as to locate the pivot at some level above the plane of the seat. Such construction is shown for example, in U.S. Pat. No. 3,934,929. The German No. 1,248,860 illustrates still another arrangement for eliminating the relative movement between the patient back and the backrest of the dental chair during articulation of the backrest. In this patent, the backrest has a sliding connection to the seat, so that as the backrest is articulated, it can translate with respect to the seat of the dental chair. One drawback of the construction shown in this German patent is that the linkages which support the backrest must traverse an angle which is sufficiently greater than the angle of inclination traversed by the backrest. This is a disadvantage since it is known that with the common push-pull drives used in dental chairs it is difficult to control movement over large angles approaching 180.degree..
Still other prior art arrangements employ a pair of linkages for connecting the backrest to the seat. The pivot points for these linkages would be at a dead center position and when the backrest was fully horizontal so that at this extreme position it was difficult to control the movement of the backrest.